This invention is concerned with the rotary method of "Big Hole" shaft drilling wherein hydraulic fluid, usually "mud", is pumped down the drill string and onto the bottom of the hole to clean the bit rollers and to flush the chips up the well bore. A great deal of effort has been expended on bottom hole cleaning coupled with drill bit design, in order to increase penetration rate, and mud pressure and its hydrostatic head is a controlling factor in this drilling efficiency. The weight of the mud is controlled and related to the bottom hole pressure required because of the pore pressure, represented by "formation pressure gradient", that presents an instantaneous boundary pressure interface of mud-to-formation at and surrounding the drill bit. The formation pressure gradient varies with depth and type of formation entered into, and it can change rapidly and unexpectedly resulting in "blowouts" or "kicks", which has required blowout prevention equipment installed at the surface as a first line of defense.
Large diameter mine shafts, leaching holes, access shafts to oil shale and tar sands and the like, and wells in general, are being established by means of large drilling rigs operating in the conventional "Rotary Drilling" mode. The two characteristic features of rotary drilling are the presence of a drilling fluid column and the weight of the drilling string applied to the drill bit. Similarly, with large diameter "Big Hole" drilling, the drilling fluid stands in the well bore so as to apply its hydrostatic head at the bit-to-bore bottom interface, and the relatively small weight of the drilling string is augmented by the applied mass of doughnut-shaped discs analogous to a stack of coins, so as to provide the necessary weight as the drill bit turns to make the hole. In practice for example, such weights rotate with the bit and are made of cast iron each 130 inches in diameter weighing 182,000 lbs.; and a plurality of these weights are normally used totaling as much as or more than one million pounds; and all of which requires a commensurate drilling rig with great weight handling and/or lifting ability.
In view of these prior art practices, it is a general object of this invention to utilize the hydrostatic head of drilling fluid or the like standing in the well bore, and its downward force as the primary weight applied to the drill bit, and eliminating the necessity of a rotating mass heretofore applied by drill collars and the like discs as above referred to. The present invention is characterized by a packer at and over the drill bit, that applies the hydrostatic head of fluid standing in the well bore to the drill bit, while separating the bit-to-bore bottom interface from said standing hydrostatic head whereby differential drilling is made possible in accordance with the teaching of my co-pending application Ser. No. 496,133, filed May 19, 1983, entitled DIFFERENTIAL PRESSURE DRILLING TOOL METHOD AND APPARATUS FOR WELL DRILLING, refiled Dec. 26, 1984 as continuation in part application Ser. No. 686,241 entitled ANNULUS BYPASS PERIPHERAL NOZZLE JET PUMP PRESSURE DIFFERENTIAL DRILLING TOOL AND METHOD FOR WELL DRILLING.
With these factors in mind, the driller normally proceeds with control over weight and pressure to establish an "over balanced" condition by substantially equalizing or exceeding the formation pressure at the bottom of the hole, and it is generally accepted by drillers that in deep-hole drilling the bit hydraulics has a major effect upon the rate of penetration. It is also accepted that the rate of penetration can be increased greatly with "under balanced" conditions at the bit, but with the risk of cave-in and uncontrolled implosion of the well bore at and surrounding the lower end of the drill string. In view of the foregoing, it is a general object of this invention to provide a pressure differential drilling tool, method and apparatus for increasing the rate of penetration in Big Hole drilling.
The formation pore pressure at the bottom of a mine shaft or well bore varies with the depth and the type of formation, and which can be calculated by using as a factor the "formation pressure gradient" of the formation to be penetrated. The average formation pressure gradient in the Continental United States is 0.465 p.s.i. per foot of depth; and the maximum abnormal therefor is accepted as 1.0 p.s.i. per foot of depth. The "formation pressure gradient" can be defined as the compaction pressure, the pore pressure, which the formation exhibits and from which condition it can unexpectedly release as in the case of a sudden opening from the formation and into the well bore. With an under balanced condition the formation will become sensitive and tend to implode, for purposes of this invention.
As stated above, the formation pressure gradient is important in the drilling process because it is a factor which determines the need for pressure and hydrostatic head applied in order to keep the formation intact and thereby prevent collapse of the well bore. The bit nozzles or jets are another factor, as they restrict flow and control pressure. Accordingly, pump pressure, jet restriction and hydrostatic head pressure of the mud are the controlling factors employed to establish "over balanced" and "under balanced" conditions as related to the pore pressure and the formation pressure gradient thereof, it being an object of this invention to provide a drilling tool for pressure differential drilling that subjects the large diameter bit-to-bore bottom interface to an under balanced condition, while subjecting the drill string above the bit to a higher or an over balanced condition. With the present invention, the differential condition is immediate to the bit, whereby cutting action at the bit-to-bore bottom interface is under balanced for rapid penetration, and whereby the hole above the bit is at higher pressure and/or over balanced for pore pressure mud column equilibrium.
The pressure differential apparatus and method of drilling herein disclosed is characterized by the pressure separation below and above the bit, thereby establishing distinct pressure zones, it being an object of this invention to pack off the bit during the drilling operation, so that an under balanced condition can prevail at the bit-to-bore bottom interface, and so that a higher pressure or an over balanced condition can prevail above the bit. Accordingly, a packer is provided at and above the bit to partition the bore through which the packer advances as hole is made. In practice, an expansible elastomer packer is employed and through which the drill string rotates.
The present invention requires the expanded packer to slideably engage the well bore closely behind the drill bit, it being an object of this invention to advantageously utilize the packer for the principle purpose of weight application, and also for stabilization. In the normal practice of Big Hole rotary drilling, the drill string weight is insufficient and consequently a large number of disc-shaped weights are used together with winged stabilizers, in order to make straight hole. With the present invention, the packer close above the drill bit inherently applies the weight of the hydrostatic head of fluid standing in the well bore and also centers the bit in the well bore for concentric bit-to-bore interface engagement. In practice, this packer-stabilizer slides down the well bore substantially without rotation and turns on the drill pipe or pipes with anti-friction thrust bearings, as will be described.
The state of the art employs dual wall or concentric drill pipe that separates the active drilling fluid from the static fluid standing within the large diameter well bore. For example, well shafts 20 feet in diameter are currently bored to 3,000 foot depth; 10 foot diameter shafts to 5,000 foot depth, etc.; utilizing an outside drill pipe up to 20 inches in diameter. The inside drill pipe of the dual string is approximately half the diameter of the outside drill pipe, forming an annulus for the flow of well fluid. The torque requirement for a 20 foot diameter drill bit is considered to be approximately 500,000 foot pounds, and this torque is accommodated chiefly by the outside drill pipe that is coupled to the drill bit as will be described.
A feature of Big Hole drilling is the confinement of circulating fluid to the dual drill pipe, with mud pump pressure applied through the inner drill pipe and return fluid and chips through the annulus between the two drill pipes, or vice versa. Aerated drilling fluid is also employed as an air lift for efficient removal of the huge volume of chips or cuttings produced by the Big Hole bits. Accordingly, it is an object of this invention to advantageously employ aerated drilling fluid under mud pump pressure to actuate the aforesaid packer, and alternately isolated from the said packer when it is weight set, and in both instances isolated so that full effect of the hydrostatic head in the well bore annulus surrounding the dual drill pipe string is effective as weight upon the packer and applied to the bit-to-bore bottom interface for effective drilling.
It is an object of this invention to control a large diameter packer during lowering of the drill string and also during the drilling operations, and to this end the packer is expansible in response to the application of weight or hydraulic pressure, and specifically through the application of mud pump pressure. With one form of the present invention, the packer is weight set by the drill bit setting on the bore bottom. With another form of the present invention, the packer is hydraulically operated in response to circulating fluid pressure. The cutter cleaning jets of the drill bit are preferably supplied with fluid from the well bore annulus surrounding the dual drill pipe.
A primary object of this invention is to isolate the operational fluid pressure above and below the drill bit, to generate a differential pressure in the separated and distinct zones below and above the packer, and to establish an effective hydrostatic head upon the packer that applies the necessary weight conducive to efficient drilling. Further, said distinct zones below and above the drill bit are isolated from the aforesaid pressure and return flow of circulating fluid within and through the dual drill pipes. In the basic form aerated circulating fluid is sufficient to flush the cuttings or chips. However, in the preferred and improved form the drill bit and drill string apparatus is essentially mechanical-hydraulic in its operational functions, and it is the inherent presence of dynamic fluid under pressure that is employed herein to generate the differential pressure that establishes a significant under balanced condition distinct from a balanced or over balanced condition. A feature of the present invention is the inclusion therein of at least one or more jet pumps operable in the well bore occupied by the aforesaid packer. Either aerated or non-aerated circulating fluid is employed to operate the jet pump, the aerated fluid having its advantageous effect to accelerate flushing up the dual drill pipe. The jet pump per se operates without moving parts and is capable of lifting a greater volume of fluid than is required therefor through its ejector for operation. It is the differential in pressure between the intake plenum and discharge venturi with which this invention is primarily concerned, a differential calculated to reach substantial proportion.
An object of this invention is to cooperatively combine a Big-Hole drill bit with an overlying and surrounding packer, and in one form of the invention advantageously utilizing the drill pipe annulus for the jet pump plenum, venturi and diffuser, and including the ejector of the pump therein. Characteristically, the dual drill pipe is embodied in the tool and has a through passage configuration that passes drilling chips through the drill pipe annulus to be carried away by the accelerated upward flow of aerated drilling fluid therethrough. A feature is that the jet pump revolves within the surrounding packer when it is set. Alternately, the packer is either pressure set or weight set, being pressure set by the application of circulating fluid pressure, and being weight set by the application of drill pipe weight, all as later described.
Another object of this invention is to cooperatively combine a Big Hole drill bit with an overlying and surrounding packer, and in one form of the invention advantageously utilizing the center pipe of the dual pipe drill string for the jet pump plenum, venturi and diffuser, and including the ejector of the pump therein. Characteristically, the dual drill pipe is embodied in the tool and has a through passage configuration that passes drilling chips through the center drill pipe to be carried away by the upward flow of said drilling fluid therethrough. A feature is that the jet pump revolves within the surrounding packer when it is set. Alternately, the packer is either pressure set or weight set, being pressure set by the application of circulating fluid pressure, and being weight set by the application of drill pipe weight, all as later described.
It is still another object of this invention to isolate the hydrostatic head of pressure in the annulus surrounding the aforesaid dual drill pipe from the drilling fluid circulated therethrough, and advantageously utilizing said annulus fluid surrounding the dual drill pipe to supply the bit jets with fluid for flushing the cuttings or chips. It is also an object, in this respect of fluid isolation, to utilize the circulating fluid pressure solely for the establishment of differential pressure at the bit-to-bore bottom interface with simultaneous induction flow of fluid from the hydrostatic head thereof in the well bore annulus surrounding the dual drill pipe. In practice, the hydrostatic head of fluid under pressure in the well bore above the packer is by-passed into the body of the drill bit where is supplies the cutter cleaning bit jets, as will be described.
The Big-Hole packer tool and/or packer-jet pump tool that I provide is a basic element of the well or shaft drilling apparatus of the present invention, and its utility as thus far described is associated directly with the drill bit for increasing the rate of making hole. However, it is to be understood that there are other apparatus features and combinations thereof wherein this hole packing tool and/or differential pressure packing tool can be usefully employed. It is the isolation of well fluids and the differential pressure control thereof which is of particular concern and to this end it is an object to selectively apply and remove hydraulic pressures to and from the packer, and accordingly I employ valve means operable by a "go-devil", or "knock-down plug" or "pump-down plug" or "wire line", as circumstances require. Vibration damping is inherent with this packer and drill bit combination, due to the frictional engagement of the packer with the well bore and the large mass of fluid in the hydrostatic head that restricts resonance in the smaller mass of the tool. It is also inherent with this packer as it is disclosed herein to operate it as a stabilizer, and accordingly the actuated packer centers the bit in the well bore for making straight hole. It is still a further object to separately actuate this basic differential pressure packing tool whereby under balanced drilling conditions beneath the bit can be immediately changed to over balanced conditions. Operation of this basic pressure differential packing tool at the bit can serve as an immediate "first line of defense" against "blow-outs" and "kicks", stopping blow-outs and kicks at their source. The basic packer as it is disclosed herein is set to slide within set casings and the like.